1. Field of the Invention
The present invention concerns a torsional damper comprising at least two coaxial parts mounted to rotate relative to one another within a defined range of relative angular movement and circumferentially acting elastic means between said parts resisting such relative angular movement.
2. Description of the Prior Art
This type of torsional damper is normally incorporated in the design of a friction clutch plate, particularly for automotive vehicles, in which one of the rotatable parts carries a friction disk designed to rotate with a first shaft, in practice a driving shaft and the motor output shaft in the case of an automotive vehicle, whereas another of said rotatable parts is carried on a hub designed to rotate with a second shaft, in practice a driven shaft and the gearbox input shaft in the case of an automotive vehicle.
This type of device is used to permit regulated transmission of rotational torque applied to one of its rotatable parts where the other is itself subject to a rotational torque. It is thereby able to filter vibrations which may arise at any point in the kinematic system in which it is incorporated, extending from the motor to the driven road wheels in the case of an automotive vehicle.
It is also known that it is advantageous in at least some applications, especially those relating to friction clutches for automotive vehicles, for the torque transmitted between the rotatable parts to remain low at low values of relative angular displacement therebetween.
This arrangement implies the use of circumferentially acting elastic means of relatively low stiffness for low values of torque, and provides for eliminating gearbox noise at the "dead point", with the vehicle stationary, especially when warmed up.
It would appear that to some extent at least it is desirable, from this point of view, that the range of relative angular movement during which these elastic means of relatively low stiffness act should be as extensive as possible.
These elastic means evidently saturate quickly, however, at a low value of torque.
For example, when a vehicle is in motion with the clutch engaged, if the pressure exerted on the accelerator to secure operation with increasing torque is momentarily released, so that the torque between the two rotatable parts constituting the friction clutch of the vehicle changes direction and operation is then with decreasing torque, then one of the rotatable parts immediately moves to another position relative to the other.
This movement corresponds to the taking up of the play between the two rotatable parts in question due to saturation of the elastic means of relatively low stiffness acting between them, and occurs first in one direction, on releasing the pressure on the accelerator, then in the other direction when the accelerator is again pressed.
It is accompanied by a two-fold effect which increases with the extent of the corresponding range of relative angular movement and is inversely proportional to the gear ratio selected.
The first effect is a noise, rather like a double slap and commonly referred to as a "clunk".
There is then an oscillation effect affecting the whole of the vehicle.
For similar reasons, this "clunk" occurs also on changing gear.
In fact, it occurs each time the torque on the shaft driving the gearbox begins to increase again.
Thus it also occurs with the vehicle at rest, for low rotational speeds of the friction clutch, when the accelerator is released and/or pressed.
In French Patent Application No. 79 04719 filed Feb. 23, 1979, now French Pat. No. 2,449,828 published Sept. 19, 1980, and from which priority is claimed in U.S. application Ser. No. 123,458 filed Feb. 21, 1980, it is proposed to equip a torsional damper of this type, specifically a friction clutch plate with a damper type hub, with at least one intermediate member which is responsive to centrifugal force and moves against the action of return means. For a first range of rotational speeds of the damper assembly it moves reversibly between a standby position, corresponding to relatively low rotational speeds and in which it is inoperative, and, beyond a defined rotational speed conditioned by the aforementioned return means, an operative position which therefore corresponds to rotational speeds higher than the preceding speeds and in which it bears circumferentially on one of said parts, either directly or indirectly, to modify the operating chacteristics of the damper for at least part of the range of relative angular movement.
According to the disclosure of this French Patent, for a torsional damper with only two rotating parts the intermediate member may, for example, when in its operative position provide a circumferential bearing surface for at least one of the circumferentially acting elastic means between the two rotatable parts, sufficient to secure the required modification of the operating characteristics of the damper. In practice, the elastic means of relatively low stiffness are then prevented from acting alone for low values of torque.
In a variant for a torsional damper comprising three rotatable parts the intermediate member in its operative position provides positive circumferential bearing engagement for two of said rotatable parts, one on the other, in practice the first two to come into action. Thus the circumferentially acting elastic means disposed between them, in practice elastic means of relatively low stiffness, are disabled so that the operating characteristics of the damper are modified.
In this French Patent the intermediate member remains in its standby position to a relatively high rotational speed of the damper assembly, of the order of 900 to 1 100 revolutions per minute, corresponding to the maximum motor idling speed. In this standby position the intermediate member permits the circumferentially acting elastic means which it controls to act freely so as to achieve the required filtering of dead point noise, and it is only at higher rotational speeds that, under the effect of centrifugal force, it moves to its operative position and so modifies the operating characteristics of the damper.
This arrangement is perfectly suitable for eliminating the "clunk" noise when in motion and the normal oscillatory movement observed on releasing pressure on the accelerator.
It is insufficient, however, to eliminate the "clunk" noise with the vehicle stopped, which occurs at rotational speeds of the clutch assembly below those for which filtering of the dead point noise must be provided.
The objective of the present invention is to provide an intermediate member which is able to eliminate this "clunk" noise with the vehicle stopped.